block: move tag field position in struct request

__data_len and __sector are internal fields which should not be accessed
directly in driver-level like the comment above it. But, tag field can
be accessed by driver level directly so that we need to make the comment
right by moving it to some other place.

Cc: Jens Axboe <axboe@kernel.dk>
Cc: linux-block@vger.kernel.org
Signed-off-by: NMinwoo Im <minwoo.im.dev@gmail.com>
Reviewed-by: NChaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

include/linux/blkdev.h

@@ -137,11 +137,11 @@ struct request {
 	unsigned int cmd_flags;		/* op and common flags */
 	req_flags_t rq_flags;
 
+	int tag;
 	int internal_tag;
 
 	/* the following two fields are internal, NEVER access directly */
 	unsigned int __data_len;	/* total data len */
-	int tag;
 	sector_t __sector;		/* sector cursor */
 
 	struct bio *bio;